This invention relates to methods and arrangements for controlling the wave shape in permanent magnet machines.
In certain permanent magnet machines such as alternators harmonics can be injected into the output to vary the wave shape by adding an external transformer to the system. For this purpose an external transformer has a secondary winding connected in series with the output from the alternator and a primary winding which is energized from an external source such as a signal generator/linear power amplifier combination or by a small alternator that generates a high frequency output. External transformers in such systems are typically large and moderately expensive devices that are designed to suit particular applications. For low frequency operation the transformers can be even larger.
Linear power amplifiers generally have poor efficiency and must have appreciable size if used for introducing significantly large harmonics. In such systems an external power source is necessary for the power amplifier and the signal generator has to be synchronized in phase with the power line for the main alternators. If a small alternator is used instead of a signal generator/power amplifier combination, it must also be mechanically synchronized to the main alternator, for example, by being mounted on a common shaft with the main alternator. Because of its high cost, complex set-up and limited flexibility such external transformer arrangements are seldom used except for laboratory applications.
It is also possible to create a variety of wave shapes by using a single or multiphase inverter with an electronic control system. Such arrangements convert a direct current link voltage to a single phase or multiphase AC voltage output with virtually any desired harmonic content. Inverters are electronic devices that can approximate wave shapes by the rapid switching of a DC voltage which generates high frequency harmonics that must be filtered out in many applications. Inverters require the use of a DC source for power and consequently they must be connected either to a DC power supply or to an AC power supply having a rectified output. This additional circuitry increases cost, size and complexity to the system. For these reasons, inverters are normally used in applications in which a source with variable voltage and frequency output is necessary and wave shape control is not a primary objective.
The Akemakou U.S. Pat. Nos. 6,037,691, 6,072,257, 6,093,992 and 6,147,429 discloses permanent magnet machines having a rotor containing permanent magnets and a core with a control coil for controlling the amount of flux from the permanent magnets passing into or avoiding paths extending through the surrounding stator coils. The control coils are energized to increase or decrease the flux from the permanent magnets linking the stator coils to control the output voltage.
The Lipo et al. U.S. Pat. No. 5,825,113 discloses a permanent magnet machine having permanent magnets in the stator together with field windings for increasing or decreasing the flux generated by the permanent magnets to control the output torque of the rotor driven by power supplied to the stator when the machine is operated as a motor or the power generated when the machine is operated as a generator.
The patent to Gokhale U.S. Pat. No. 4,885,493 discloses a permanent magnet alternator with a flux diversion member resiliently coupled to the permanent magnet rotor to permit variation of the relative rotary position of the flux diversion member so as to control the alternator output voltage.
The Byrne U.S. Pat. No. 3,508,094 discloses a permanent magnet AC generator in which the shape of the output wave is modified by providing screened zones between the permanent magnets in the rotor in which there is substantially no radial flux density from the permanent magnet poles.
Accordingly, it is an object of the present invention to provide a wave shape control arrangement for permanent magnet machines which overcomes disadvantages of the prior art.
Another object of the invention is to provide a permanent magnet machine capable of providing an output having a desired wave shape.
These and other objects of the invention are attained by providing a permanent magnet machine having a rotor containing permanent magnets and flux shunts containing control windings along with a control unit for controlling the flux in the shunts based on differences between the wave form of the output and a reference wave form in order to control the wave shape of the output from the machine. In a preferred arrangement, current is supplied to the flux shunt windings from a rotary transformer or exciter that rotates with the rotor and the AC power generated in the exciter or rotating transformer is converted to DC power by rectifiers positioned on the rotating assembly. To control the current supplied to the flux shunt windings a controller senses the output wave form from the machine and compares it with the reference wave form to produce a difference signal to adjust the flux shunt current.